The orientation effect of gramicidin A on bicelles and Eu3+-doped bicelles as studied by solid-state NMR and FT-IR spectroscopy

Morphological control and modern applications of bicelles

Bicellar systems have become popularized as their rich morphology can be applied in biochemistry, physical chemistry, and drug delivery technology. To the biochemical field, bicelles are powerful model membranes for the study of transmembrane protein behavior, membrane transport, and environmental interactions with the cell. Their morphological responses to environmental changes reveal a profound fundamental understanding of physical chemistry related to the principle of self-assembly. Recently, they have also drawn significant attention as theranostic nanocarriers in biopharmaceutical and diagnostic research due to their superior cellular uptake compared to liposomes. It is evident that applications are becoming broader, demanding to understand how the bicelle will form and behave in various environments. To consolidate current works on the bicelle's modern applications, this review will discuss various effects of composition and environmental conditions on the morphology, phase behavior, and stability. Furthermore, various applications such as payload entrapment and polymerization templating are presented to demonstrate their versatility and chemical nature.

Investigating the action of the microalgal pigment marennine on Vibrio splendidus by in vivo2H and 31P solid-state NMR

This work investigates the potential probiotic effect of marennine - a natural pigment produced by the diatom Haslea ostrearia - on Vibrio splendidus. These marine bacteria are often considered a threat for aquaculture; therefore, chemical antibiotics can be required to reduce bacterial outbreaks. In vivo²H solid-state NMR was used to probe the effects of marennine on the bacterial membrane in the exponential and stationary phases. Comparisons were made with polymyxin B (PxB) - an antibiotic used in aquaculture and known to interact with Gram(-) bacteria membranes. We also investigated the effect of marennine using ³¹P solid-state NMR on model membranes. Our results show that marennine has little effect on phospholipid headgroups dynamics, but reduces the acyl chain fluidity. Our data suggest that the two antimicrobial agents perturb V. splendidus membranes through different mechanisms. While PxB would alter the bacterial outer and inner membranes, marennine would act through a membrane stiffening mechanism, without affecting the bilayer integrity. Our study proposes this microalgal pigment, which is harmless for humans, as a potential treatment against vibriosis.

NMR structural studies and mechanism of action of Lactophoricin analogs as antimicrobial peptides

Antimicrobial peptides (AMPs) are effective alternatives to conventional antibiotics. They protect the host from the constant invasion of a broad range of infectious microorganisms. AMPs have been at the forefront of the response to multidrug-resistant microbial strains and appear to be ideal drug candidates. Lactophoricin (LPcin), naturally produced from bovine milk, is a typical cationic antimicrobial peptide. Three analog peptides, including LPcin-YK5, LPcin-YK8, and LPcin-YK11, with enhanced antimicrobial activity compared to the wild-type LPcin, were designed and expressed in our laboratory. We investigated the structure and antimicrobial mechanisms of action of the three novel antimicrobial peptide analogs derived from LPcin using solution NMR and solid-state NMR spectroscopy in membrane environments. Our results revealed that the three LPcin analogs exhibited helical structures with different tilt angles on the phospholipid membrane surface. We proposed three-dimensional conformations and antibacterial mechanisms of action of the three peptide analogs in the phospholipid bilayers using two-dimensional solid-state separated local field NMR experiments.

Effects of Amphipathic Polypeptides on Membrane Organization Inferred from Studies Using Bicellar Lipid Mixtures

SP-B_63-78, a lung surfactant protein fragment, and magainin 2, an antimicrobial peptide, are amphipathic peptides with the same overall charge but different biological functions. Deuterium nuclear magnetic resonance has been used to compare the interactions of these peptides with dispersions of 1,2-dimyristoyl- sn-glycero-3-phophocholine (DMPC)/1,2-dihexanoyl- sn-glycero-3-phophocholine (DHPC) (4:1) and DMPC/1,2-dimyristoyl- sn-glycero-3-phopho-(1'-rac-glycerol) (DMPG)/DHPC (3:1:1), two mixtures of long-chain and short-chain lipids that display bicellar behavior. This study exploited the sensitivity of a bicellar system structural organization to factors that modify partitioning of their lipid components between different environments. In small bicelle particles formed at low temperatures, short-chain components preferentially occupy curved rim environments around bilayer disks of the long-chain components. Changes in chain order and lipid mixing, on heating, can drive transitions to more extended assemblies including a magnetically orientable phase at intermediate temperature. In this work, neither peptide had a substantial effect on the behavior of the zwitterionic DMPC/DHPC mixture. For bicellar mixtures containing the anionic lipid DMPG, the peptide SP-B_63-78 lowered the temperature at which magnetically orientable particles coalesced into more extended lamellar structures. SP-B_63-78 did not promote partitioning of the zwitterionic and anionic long-chain lipid components into different environments. Magainin 2, on the other hand, was found to promote separation of the anionic lipid, DMPG, and the zwitterionic lipid, DMPC, into different environments for temperatures above 34 °C. The contrast between the effects of these two peptides on the lipid mixtures studied appears to be consistent with their functional roles in biological systems.

Bicelle composition-dependent modulation of phospholipid dynamics by apelin peptides 1

Apelin peptides are cognate ligands for the apelin receptor, a G-protein-coupled receptor (GPCR). The apelinergic system plays critical roles in wide-ranging physiological activities including function and development of the central nervous and cardiovascular systems. Apelin is found in 13-55 residue isoforms in vivo, all of which share the C-terminal portion of the preproapelin precursor. Characterization of high-resolution structures and detergent micelle interactions of apelin-17 led to a two-step membrane-catalyzed binding and GPCR activation mechanism hypothesis recapitulated in longer isoforms. Here, we examine interactions of the apelin-13 and -17 isoforms with isotropic zwitterionic and mixed zwitterionic-anionic lipid bicelles to test for hallmarks of membrane catalysis in a more physiological membrane-mimetic environment than a micelle. Specifically, ¹H and ³¹P relaxation and diffusion solution-state NMR techniques demonstrate that both apelin isoforms interact with both types of isotropic bicelles. Bicelle hydrodynamics were observed to be differentially modulated by apelin peptides, although these effects were minimal. Phospholipid headgroup ³¹P spin relaxation behaviour was, conversely, clearly perturbed. Perturbation of this nature was also observed in magnetically aligned bicelles by ³¹P solid-state NMR spectroscopy and spin relaxation experiments. This behaviour is consistent with an apelin-bicelle binding process allowing significant peptide mobility, facilitating membrane-catalyzed GPCR encounter.

Membrane solid-state NMR in Canada: A historical perspective

This manuscript presents an overview of more than 40years of membrane solid-state nuclear magnetic resonance (NMR) research in Canada. This technique is a method of choice for the study of the structure and dynamics of lipid bilayers; bilayer interactions with a variety of molecules such as membrane peptides, membrane proteins and drugs; and to investigate membrane peptide and protein structure, dynamics, and topology. Canada has a long tradition in this field of research, starting with pioneering work on natural and model membranes in the 1970s in a context of emergence of biophysics in the country. The 1980s and 1990s saw an emphasis on studying lipid structures and dynamics, and peptide-lipid and protein-lipid interactions. The study of bicelles began in the 1990s, and in the 2000s there was a rise in the study of membrane protein structures. Novel perspectives include using dynamic nuclear polarization (DNP) for membrane studies and using NMR in live cells. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Probing topology and dynamics of the second transmembrane domain (M2δ) of the acetyl choline receptor using magnetically aligned lipid bilayers (bicelles) and EPR spectroscopy

Characterizing membrane protein structure and dynamics in the lipid bilayer membrane is very important but experimentally challenging. EPR spectroscopy offers a unique set of techniques to investigate a membrane protein structure, dynamics, topology, and distance constraints in lipid bilayers. Previously our lab demonstrated the use of magnetically aligned phospholipid bilayers (bicelles) for probing topology and dynamics of the membrane peptide M2δ of the acetyl choline receptor (AchR) as a proof of concept. In this study, magnetically aligned phospholipid bilayers and rigid spin labels were further utilized to provide improved dynamic information and topology of M2δ peptide. Seven TOAC-labeled AchR M2δ peptides were synthesized to demonstrate the utility of a multi-labeling amino acid substitution alignment strategy. Our data revealed the helical tilts to be 11°, 17°, 9°, 17°, 16°, 11°, 9°±4° for residues I7TOAC, Q13TOAC, A14TOAC, V15TOAC, C16TOAC, L17TOAC, and L18TOAC, respectively. The average helical tilt of the M2δ peptide was determined to be ∼13°. This study also revealed that the TOAC labels were attached to the M2δ peptide with different dynamics suggesting that the sites towards the C-terminal end are more rigid when compared to the sites towards the N-terminus. The dynamics of the TOAC labeled sites were more resolved in the aligned samples when compared to the randomly disordered samples. This study highlights the use of magnetically aligned lipid bilayer EPR technique to determine a more accurate helical tilt and more resolved local dynamics of AchR M2δ peptide.

Phase behavior of a binary lipid system containing long- and short-chain phosphatidylcholines

A new phase state, named the U phase, was observed in DPPC–diC8PC mixtures at low DPPC contents.

Oriented samples: a tool for determining the membrane topology and the mechanism of action of cationic antimicrobial peptides by solid-state NMR

Overuse and misuse of antibiotics have led bacteria to acquire several mechanisms of resistance. In response to this, researchers have identified natural antimicrobial peptides as promising candidates to fight against multiresistant bacteria. However, their mode of action is still unclear, and a better understanding of the mode of action of these peptides is of primary importance to develop new peptides displaying high antibacterial activity and low hemolytic activity. One of the main features that defines the mechanism of action is the membrane topology of the peptide. Among the spectroscopic techniques, solid-state NMR is the technique of choice for determining the location of the peptide within the membrane. It can be achieved by performing experiments with oriented samples. In the literature, the two most common types of oriented samples are bicelles and phospholipids mechanically oriented between glass plates. The mode of perturbation of the membrane-active peptide can be studied by phosphorus-31 and deuterium NMR. On the other hand, several experiments such as nitrogen-15 and fluorine solid-state NMR, that require labeled peptides, can give valuable information on the membrane topology of the peptide. The combination of the latter techniques allows the determination of a precise topology, thus a better knowledge of the molecular determinants involved in the membrane interactions of antimicrobial peptides.

Distinguishing gramicidin D conformers through two-dimensional infrared spectroscopy of vibrational excitons

Gramicidin D is a short peptide which dimerizes to form helical pores, adopting one of two conformations in the process. These conformations differ primarily in number of residues per turn and the hydrogen-bond registry between rungs of the helix. Using amide I 2D infrared (IR) and FTIR, we have demonstrated that it is possible to distinguish between the different conformers of gramicidin D in solution. We show that the spectra observed for this helical peptide bear no resemblance to the spectra of α- or 310-helices and that while the FTIR spectra appear similar to spectra of β-sheets, 2D IR reveals that the observed resonances arise from vibrational modes unlike those observed in β-sheets. We also present an idealized model which reproduces the experimental data with high fidelity. This model is able to explain the polarization-dependence of the experimental 2D IR data. Using this model, we show the coupling between the rungs of the helix dominates the spectra, and as a consequence of this, the number of residues per turn can greatly influence the amide I spectra of gramicidin D.

Solid-state nuclear magnetic resonance spectroscopy for membrane protein structure determination

Solid-state NMR (ssNMR) is a versatile technique that can provide high-resolution (sub-angstrom) structural data for integral membrane proteins embedded in native and model membrane environments. The methodologies for a priori structure determination have for the most part been developed using samples with crystalline and fibrous morphologies. However, the techniques are now being applied to large, polytopic membrane proteins including receptors, ion channels, and porins. ssNMR data may be used to annotate and refine existing structures in regions of the protein not fully resolved by crystallography (including ligand-binding sites and mobile solvent accessible loop regions). This review describes the spectroscopic experiments and data analysis methods (including assignment) used to generate high-resolution structural data for membrane proteins. We also consider the range of sample morphologies that are appropriate for study by this method.

Fully deuterated magnetically oriented system based on fatty acid direct hexagonal phases

There is strong demand in the field of NMR for simple oriented lipid supramolecular assemblies, the constituents of which can be fully deuterated, for specifically studying the structure of host protonated molecules (e.g., peptides, proteins...) in a lipid environment. Also, small-angle neutron scattering (SANS) in fully deuterated oriented systems is powerful for gaining information on protonated host molecules in a lipid environment by using the contrast proton/deuterium method. Here we report on a very simple system made of fatty acids (dodecanoic and tetradecanoic) and ethanolamine in water. All components of this system can be obtained commercially as perdeuterated. Depending on the molar ratio and the concentration, the system self-assembles at room temperature into a direct hexagonal phase that is oriented by moderate magnetic fields of a few tesla. The orientation occurs within the magnetic field upon cooling the system from its higher-temperature isotropic phase: the lipid cylinders of the hexagonal phase become oriented parallel to the field. This is shown by solid-state NMR using either perdeuterated fatty acids or ethanolamine. This system bears strong interest for studying host protonated molecules but also in materials chemistry for building oriented solid materials.

A novel oriented system made of fatty acid hexagonal phases with tuneable orientation

There is a strong demand in the field of solid state NMR for oriented lipid supramolecular assemblies. This is mainly devoted to biophysical structural studies or materials chemistry because the NMR signal depends on the orientation. Here we report a novel system made of a fatty acid hexagonal phase which self orient in the magnetic field. The orientation occurs within the magnetic field upon cooling the system from its isotropic phase. The cylinders of the hexagonal phase are then oriented parallel to the field. We take advantage that the hexagonal phase is a gel, i.e., the orientation is maintained fixed within the sample tube to investigate the orientational dependence of the deuterium solid state NMR signal using deuterated fatty acids and D(2)O by manually rotating the sample tube within the coil probe. As expected, the oriented signal follows the low |3cos(2)theta-1| where theta is the angle between the long cylindrical axis and the field. We expect this system to be of interest in materials chemistry and structural biology.

Chapter 10 - Lanthanide-loaded paramagnetic liposomes as switchable magnetically oriented nanovesicles

Magnetically oriented liposomes can be prepared by exposing unilamellar spherical systems loaded with paramagnetic lanthanide(III) complexes to hyperosmotic stress. The resulting nonspherical, lens-shaped, nanoparticles can orient within a static magnetic field, depending on the magnetic properties of their membrane bilayer. The orientation of the vesicles can be easily determined by measuring the paramagnetic contribution to the (1)H chemical shift of the intraliposomal water proton resonance. As the latter shift is dominated by the bulk magnetic susceptibility contribution, its sign (negative or positive) reports about the preferred orientation adopted by the nanovesicles. The alignment within the field depends upon the magnetic susceptibility anisotropy of the liposome membrane, Delta(chi)(LIPO). When Delta(chi)(LIPO) has a negative value (e.g., for nonspherical liposomes made of conventional phospholipids), the nanoparticles align with their long axis parallel to the field, whereas when Delta(chi)(LIPO)>0 the vesicles flip by 90 degrees . The sign of the chemical shift of the intraliposomal water resonance is positive in the former case and negative in the latter, respectively. The liposome orientation can be switched by incorporating in the liposome bilayer suitable amphiphilic paramagnetic lanthanide(III) complexes. The sign of Delta(chi)(LIPO), and consequently the magnetic alignment, will correspond to the sign of the magnetic susceptibility anisotropy of the metal complex. The magnetic susceptibility anisotropy is dependent on both the electronic configuration of the lanthanide ion and the structural characteristics of the amphiphilic complex incorporated in the liposome membrane. The magnetic orientation of such vesicles is maintained in vivo, thus opening promising perspectives for the application of nonspherical liposomes in medical imaging.

Determining the topology of integral membrane peptides using EPR spectroscopy

This paper reports on the development of a new structural biology technique for determining the membrane topology of an integral membrane protein inserted into magnetically aligned phospholipid bilayers (bicelles) using EPR spectroscopy. The nitroxide spin probe, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC), was attached to the pore-lining transmembrane domain (M2delta) of the nicotinic acetylcholine receptor (AChR) and incorporated into a bicelle. The corresponding EPR spectra revealed hyperfine splittings that were highly dependent on the macroscopic orientation of the bicelles with respect to the static magnetic field. The helical tilt of the peptide can be easily calculated using the hyperfine splittings gleaned from the orientational dependent EPR spectra. A helical tilt of 14 degrees was calculated for the M2delta peptide with respect to the bilayer normal of the membrane, which agrees well with previous 15N solid-state NMR studies. The helical tilt of the peptide was verified by simulating the corresponding EPR spectra using the standardized MOMD approach. This new method is advantageous because: (1) bicelle samples are easy to prepare, (2) the helical tilt can be directly calculated from the orientational-dependent hyperfine splitting in the EPR spectra, and (3) EPR spectroscopy is approximately 1000-fold more sensitive than 15N solid-state NMR spectroscopy; thus, the helical tilt of an integral membrane peptide can be determined with only 100 microg of peptide. The helical tilt can be determined more accurately by placing TOAC spin labels at several positions with this technique.

Effects of lipid chain length and unsaturation on bicelles stability. A phosphorus NMR study

Most studies reported until now on the magnetically alignable system formed by the binary mixtures of long- and short-chain lipids were based on the mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (D14PC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (D6PC) lipids. We have recently shown that a large part of the phase diagrams of this lipid mixture could be understood by taking into account the partial miscibility between the long-chain lipids and the short-chain lipids when the sample was heated above the melting transition temperature (Tm) of the long-chain lipids. In this work, we show by modifying the chain length of either one of the two lipids that it is possible to control their miscibility and thus the intervals of temperature and composition where spontaneous alignment is observed in a magnetic field. By using 31P NMR, we demonstrate that the very special properties of such binary lipid mixtures are correlated with the propensity for short-chain lipids to diffuse into the bilayer regions. We also show that lipid mixtures with comparable properties can be formed with unsaturated lipids such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC).